internal/coverage: add coverage meta-data encoder

Add a new package with APIs for encoding coverage meta-data. This
provides support for accumulating information about each function
during the compilation process, and then encoding and emitting a
payload for a coverage meta-data symbol.  Not yet connected to the
rest of the coverage machinery (that will appear in a later patch).

Updates #51430.

Change-Id: I61054ce87f205b25fb1bfedaa740fd7425c34de4
Reviewed-on: https://go-review.googlesource.com/c/go/+/353453
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/internal/coverage/stringtab/stringtab.go

@@ -0,0 +1,89 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package stringtab
+
+import (
+	"fmt"
+	"internal/coverage/uleb128"
+	"io"
+)
+
+// This package implements a string table writer utility for use in
+// emitting coverage meta-data and counter-data files.
+
+type Writer struct {
+	stab   map[string]uint32
+	strs   []string
+	tmp    []byte
+	frozen bool
+}
+
+// InitWriter initializes a stringtab.Writer.
+func (stw *Writer) InitWriter() {
+	stw.stab = make(map[string]uint32)
+	stw.tmp = make([]byte, 64)
+}
+
+// Nentries returns the number of strings interned so far.
+func (stw *Writer) Nentries() uint32 {
+	return uint32(len(stw.strs))
+}
+
+// Lookup looks up string 's' in the writer's table, adding
+// a new entry if need be, and returning an index into the table.
+func (stw *Writer) Lookup(s string) uint32 {
+	if idx, ok := stw.stab[s]; ok {
+		return idx
+	}
+	idx := uint32(len(stw.strs))
+	stw.stab[s] = idx
+	stw.strs = append(stw.strs, s)
+	return idx
+}
+
+// Size computes the memory in bytes needed for the serialized
+// version of a stringtab.Writer.
+func (stw *Writer) Size() uint32 {
+	rval := uint32(0)
+	stw.tmp = stw.tmp[:0]
+	stw.tmp = uleb128.AppendUleb128(stw.tmp, uint(len(stw.strs)))
+	rval += uint32(len(stw.tmp))
+	for _, s := range stw.strs {
+		stw.tmp = stw.tmp[:0]
+		slen := uint(len(s))
+		stw.tmp = uleb128.AppendUleb128(stw.tmp, slen)
+		rval += uint32(len(stw.tmp)) + uint32(slen)
+	}
+	return rval
+}
+
+// Write writes the string table in serialized form to the specified
+// io.Writer.
+func (stw *Writer) Write(w io.Writer) error {
+	wr128 := func(v uint) error {
+		stw.tmp = stw.tmp[:0]
+		stw.tmp = uleb128.AppendUleb128(stw.tmp, v)
+		if nw, err := w.Write(stw.tmp); err != nil {
+			return fmt.Errorf("writing string table: %v", err)
+		} else if nw != len(stw.tmp) {
+			return fmt.Errorf("short write emitting stringtab uleb")
+		}
+		return nil
+	}
+	if err := wr128(uint(len(stw.strs))); err != nil {
+		return err
+	}
+	for _, s := range stw.strs {
+		if err := wr128(uint(len(s))); err != nil {
+			return err
+		}
+		if nw, err := w.Write([]byte(s)); err != nil {
+			return fmt.Errorf("writing string table: %v\n", err)
+		} else if nw != len([]byte(s)) {
+			return fmt.Errorf("short write emitting stringtab")
+		}
+	}
+	return nil
+}